Metabolic and nutritional evaluation of patients supported with mechanical ventilation

Aggressive nutritional support has been advocated for critically ill patients. Ten patients in a medical ICU supported with mechanical ventilation, 3 on total parenteral nutrition (TPN) and 7 not on TPN, were studied over a 24-h period for oxygen uptake (VO2), carbon dioxide production (VCO2), respiratory exchange ratio (R), and urinary urea nitrogen (UN). Indirect calorimetric formulas were used to calculate the caloric expenditure and this was compared to calories administered. The TPN group had a mean R of 1.23 which was significantly greater than the nonTPN group whose mean R was 0.88 (p < 0.001). The increased VCO2 noted in the TPN group may have been secondary to high carbohydrate intake with fat synthesis and could play a role in patients who are difficult to wean from a ventilator.